1. Field of the Invention
This invention relates to apparatus and methods for grinding tapers or points on elongated stock and more particularly for grinding points on needle stock.
2. Description of the Related Art
The production of quality needles from raw stock involves many different processes and machinery. These varying processes and machinery become more critical in the preparation of surgical needles where the environment of intended use is in humans or animals. Some of the processes involved in the production of surgical grade needles include, inter alia: straightening spooled wire stock; cutting needle blanks from raw stock; providing a bore for receiving suture thread at one end of the blank; tapering or grinding points at the other end of the blank, flat pressing a portion of the needle barrel to facilitate easier grasping by surgical instrumentation; and, where curved needles are desired, curving the needle. During each of these several steps, extreme care must be taken to ensure that only the intended working of the needle is performed and that the other parts of the needle remain undisturbed.
Machines for grinding points of needles are known. Such machines include the Type NS 6, 8, 11 and 15 automatic point grinding machines available from SCHUMAG Machinery, Inc. of Norwood, N.J. Those machines utilize, for example, a transport wheel and saddle arrangement to present wire shafts to a grinding wheel so as to grind points on the ends of the shafts. A notched wheel is provided for spacing apart the wire shafts and presenting them to the grinding wheel surface. Different sized notched wheels are required for different wire shaft diameters. Thus, in order to change diameter shafts being ground, the appropriate notched wheel must first be installed before grinding of the differing diameter shaft needle blanks can take place. This requires shutting down production and fitting the machine with the appropriate notched wheel each time stock having a diameter not appropriate for the current wheel is to be ground.
Additionally, to operate efficiently, these machines require that the minimum wire shaft length must be longer than many of the surgical needles presently in use, thus necessitating additional finishing steps to refine the dimensions of the needles. Therefore, in order to form a finished needle, for example, having a length of 0.875 inches, stock of at least 13/4 inches would have to first be ground and then be clipped to the desired length. By performing the grinding first and then having a clip the needle shaft to the desired length, chances are increased that the needle point will become damaged during handling and clipping. Moreover, clipping the end of needle stock necessitates additional processing and quality control steps. Specifically, any burs or other irregularities created from the clipping must be removed and samples inspected to ensure the quality of the work.
Finally, grinding wheels employed by these known devices are typically of the bonded type which generally require frequent re-dressing. During use, the abrasive grains on bonded grinding wheels become slightly dulled. Normal stresses in the grinding operation should increase enough to tear the worn grain from the wheel to expose a new cutting grain. Thus, too soft a wheel wears too fast, losing grains before they are dulled, whereas, too hard a wheel develops a smooth glazed surface that will not cut properly. As the abrasives used on grinding wheel surfaces wear with continuous grinding, the wheel configuration changes enough to affect the grind on the finished product. When this occurs the wheel must then be re-dressed in the manner described above to open new abrasive grain surfaces or recondition the grinding surface so as to afford maximum cutting qualities. The process of dressing a grinding wheel is subjective, however, in that the dressing of a grinding wheel of the type used for grinding needle points usually requires an operator perform the dressing manually, therefore causing the geometry of the dressed wheel to vary, however slightly, from operator to operator. This variance in the grinding wheel surface causes differences in the finished needle point geometries which must conform to strict specifications. The result is a higher percentage of rejected parts and, therefore, increased operating and quality control costs.
Other methods are known for forming grinding wheels, for example, electroplating (a form of metal bonding) abrasive materials to a wheel core. The abrasive materials used for such bonding are typically abrasives such as diamond and cubic boron nitride which was developed by the General Electric Company and is available under the tradename Borazon. These specialized wheels offer many advantages not available with conventional vitrified, metal or resin bond wheels. Electroplated wheels may be custom designed to form requirements and therefore, offer immediate fast cutting as purchased. The cutting edges of super-abrasive materials do not break off as do those of conventional bonding materials. Instead, they wear down gradually over a long period of time. Therefore, grinding wheels plated with the above-mentioned abrasives provide the exact grinding surface geometry required for precision grinding without requiring dressing or re-dressing to generate and retain form. No break in period is required and wheel cores are reusable, thus reducing replacement costs. The present invention is directed toward apparatus and methods for grinding high quality surgical needles while avoiding the disadvantages of known devices.